The present invention relates to a method and implementing system for storing digital data. While the invention will be developed in relation to storage on and retrieval from magnetic surfaces, the underlying principles have broader application. However, presently the invention will find preferred utility in the field of magnetic disk storage.
Rotating, high-speed disks are very important memory extensions for digital data processing systems, as they permit storage of large quantities of data at relatively low cost. The successful employment of disk storage systems depends to a considerable extent on high data package densities that can be achieved. The limit in package densities is primarily determined by the limitations posed by the magnetic transducer-disk surface interaction.
The transducers must provide magnetic flux and flux reversals to the surface and must respond to the reversals upon retrieval. The spacing of these flux reversals on a data storage track is of digital significance and the retrieval process must distinguish among sequential reversals as to the relative time of occurrence of read back signal peaks which represent the passage of flux reversal on a disk under a pick up or read transducer. These interaction processes establish operational limitations in that flux reversals, sometimes also called transitions, when too closely spaced are no longer recognizable and are nonretrievable as to their information significance; the read back signal peaks are no longer distinguishable from noise and readily merge. Additionally, the difference in spacing of these transitions on a data track are of immediate information significance, but if some transitions are placed too close, they appear shifted upon retrieval (peak displacement), and that, in turn, tends to eradicate the informative differences in read back peak spacing. Upon retrieval of data, "windows" are generated and it is determined whether a read back signal peak does or does not fall in a particular window. The width of that window determines the resolution of the entire record/retrieval process.
Among the most commonly used data encoding techniques are those which store bivalued bits as data on a track under utilization of two different transition spacings and, therefore, of two different recording frequencies, whereby the larger spacing (smaller frequency) determined also the bit rate, and the smaller spacing (higher frequency) was usually chosen to be half the larger spacing. These codes are known under various names: Gabor, Manchester, Frequency Doubling, Phase Shift, etc. They have in common that a bit cell or bit frame had to be twice as long as the minimum distance between the flux reversals that was permissible. To state it differently, these codes used one transition per bit, frame or cell to distinguish and to separate bit cells or frames, and additional transitions were used to distinguish among bit values.
The read resolution of these known methods, i.e. the width of the window generated to detect absence or presence of a peak was, thus, equal to the smallest signal peak separation as it occurs on readback which separation, in time, was thus half of the data rate period. If that smallest peak separation as actually used is close to the minimum separation and spacing possible, or permissible, data package density is at its maximum for such a system. Any increase in package density required a change in encoding techniques (or a change in components). It should be mentioned also that it was inherent in these codes (usually developed for magnetic tapes) that they have self-clocking; a feature deemed desirable for tapes, but no longer significant for disks.
In order to increase the package density of data, other codes have been developed as to implementation in the recent past, some of which go back to a code format developed by Pouliard and others (U.S. Pat. No. 2,807,004). These codes require that at the most there be one transition per bit cell and frame, and at least one per two bit cells. The encoding required that transitions may follow at times at a spacing equal to one and one-half frame or bit cell length. Codes of that type are shown and implemented, for example, in U.S. Pat. Nos. 3,108,261; 3,235,855; 3,414,894; 3,631,428. There appears to be no common name for these codes, but the term tri-period code seems to be appropriate as, e.g., the decoder has to distinguish among three different peak and transition spacings that may occur.
The increase in package density obtainable with these codes is significant; from standpoint of minimum permissible, spatial separation of flux reversals, the data density is doubled by these tri-period codes. The read resolution (in time) is always the minimum period of time by which readback signal peaks may differ as to occurrence. That period is still equal to half a bit frame time for these tri-period codes. Since the bit frame period in the tri-period code is one half the bit frame period in Manchester and frequency doubling, the read resolution is also one half the value it was in these earlier codes. Allowing again the smallest spacing of flux reversals to approach the minimum permissible distance, the frame width is reduced to that value in the tri-period code, but the read resolution is half that value. As a consequence, the so-called peak shift phenomenon becomes more noticeable on read back. Further increase in package density, thus, requires still further codes. The same requirement is true of the read resolution which is to be made larger without decreasing package density.
In accordance with other approaches, it has been suggested to encode bit groups by patterns unique to the group, but individual bit values within the group are no longer identifyable except within and as part of the group (U.S. Pat. No. 3,508,228). This method has the drawback that for a minimum bit spacing as large as a data bit frame, the boundaries between data bit group cells will never hold a transition (or a particular position relative to the bit frame boundaries of the data track) which constraint must be considered wasteful to some extent as far as recording space is concerned.
It should be noted that the term "boundaries" has somewhat arbitrary connotation as it relates occurrence of data bits for recording, local clock or clock track and data track. Please note that in U.S. Pat. No. 3,508,228 the entire pattern could be phase-shifted by .+-. read resolution without change in any of the pertinent points under consideration.